This is presented as a simple passive switch mechanism that can be placed on a dual speaker cabinet's jack plate, and is only one possibility for switching speakers.

I have seen numerous passive parallel/stereo cab switches and multi-speaker switch arrangements, but I haven't come across any for a 2 speaker cab series/stereo switch ...so, I drew this up. It can be handy with guitar cabinets, where separate amps/signals can drive the same cabinet. Or, in other stereo applications where separation is not critical.

As shown in the attachment, this is for two 4 ohm speakers that will be 8 ohms in series or 4 ohms each in stereo mode. Other speaker values will work too, but the total impedance will vary accordingly. With a single input, use the Left jack (8 ohms). For two separate inputs (stereo), use Left and Right jacks (4 ohms ea.). Note that the Right jack is effectively disconnected in series mode. Be wary of varying load/power ratings for your speakers.